Thermoplastic elastomers formed from blends of cured rubber and polyolefins are known in the art. The structure of such materials is in the form of a matrix containing a plastic component with discrete domains of a partially or fully cured elastomeric component embedded therein. Olefin-based thermoplastic elastomers, with the structure thus described, have the advantage of being able to undergo plastic flow above the softening point of the polyolefin, and yet behave like a cured elastomer below the softening point. Thus, the materials exhibit thermoplasticity (i.e., flowing at elevated temperature under processing conditions) while also exhibiting rubber-like elasticity (i.e., recovering a substantial amount of deformation when a deforming force is removed).
Dynamic vulcanization is a process whereby the elastomeric portion of the thermoplastic elastomer is cured by heating the blend in the presence of a curative while shearing the blend to form a thermoplastic vulcanizate (TPV). Different curing methods that may be used to partially or fully cure the rubber during dynamic vulcanization include sulfur-, peroxide- and phenolic-based systems.
The extent of cure (i.e., partial or full) of the elastomeric or rubber phase is an important factor in the ultimate properties of the final composite, such that lower oil swell and higher ultimate tensile strength are observed at high states of cure, as taught in U.S. Pat. No. 4,130,535. In other words, a blend containing a fully cured elastomeric phase has improved physical properties as compared to uncured or partially cured blends. Such fully cured vulcanizates are processable as thermoplastics although they are crosslinked to a point where the rubber portions are almost or entirely insoluble in the usual solvents. The processability of a fully cured thermoplastic vulcanizate (TPV) is in direct contrast to thermoset compositions, which retain dimensional integrity at service temperatures of 200° C. or above.
Many of the commercial TPV applications use the phenolic resin cure system as disclosed in, for example, U.S. Pat. No. 4,311,628. The thermoplastic elastomers made with this vulcanization system were shown to have better (i.e., lower) compression set and oil resistance than equivalent compositions cured with peroxide or with a sulfur-based vulcanization system. Low compression set is important for a number of applications, such as gaskets and seals. Good oil resistance is important in many automotive applications.
The good compression set and oil resistance of phenolic resin cured thermoplastic elastomers, however, has overshadowed the fact that this cure system is not environmentally friendly because of formaldehyde emissions. Additionally, such products have a dark color or yellowness and are sensitive to moisture pick-up due to the phenolic moieties in the crosslinked network.
Dynamic vulcanization using peroxide to prepare thermoplastic elastomers with the elastomeric phase only partially cured was practiced early in the development of the technology. U.S. Pat. No. 3,806,558 discloses that ethylene-propylene-diene terpolymers (EPDM) could be partially cured by dynamic vulcanization in the presence of polypropylene to provide reprocessable materials with good physical properties.
The use of a peroxide curing system, while producing lighter colored products than the phenolic curing system, may cause the degradation of the propylene resin, adversely affecting physical properties of the thermoplastic elastomer. The use of certain co-agents, which may reduce this degradation, is established in the chemical literature. As an example, some fundamental considerations in the use of co-agents in peroxide curable elastomers are shown in R. Drake and J. Labriola, ACS Rubber Division Meeting, Paper No. 5, Fall 1994.
The co-agent functions by reacting with the radicals formed from decomposition of the peroxide to form free radicals on the co-agent molecule, which then mediate the crosslinking reaction. Typically, these co-agent materials contain di- or poly-unsaturation and have a readily extractable hydrogen in the alpha position to the unsaturated bonds. Examples of such materials include trimethylolpropane trimethacrylate (TMPTMA), trimethylolpropane triacrylate (TMPTA), triallylcyanurate (TAC), triallyltrimellitate (TATM), ethyleneglycol dimethacrylate (EGDMA), triallylisocyanate (TAIC) and 1,2-polybutadiene (PBD), which is usually employed as an atactic low molecular weight liquid.
U.S. Pat. No. 4,108,947 discloses a partially cured thermoplastic elastomer composition containing an olefinic rubber and a polyolefin resin and having a cross-linking degree of less than 90% where the curing system is peroxide and a co-agent. The patent further discloses that it is important not to fully crosslink the rubber phase so that poor fluidity can be avoided.
It is also known that certain combinations of co-agents can be used with peroxides. U.S. Pat. No. 4,948,840, for example, discloses a partially cured thermoplastic elastomer containing propylene resin and fully saturated elastomers, along with a curing system containing 1,2-polybutadiene and an organic peroxide. The curing system may further contain certain additional co-agents, such as phenylene-bis-maleimide.
U.S. Pat. No. 6,207,746 discloses a partially cured thermoplastic elastomer containing polypropylene, an ethylene/olefin copolymer and a processing oil, where the ethylene/olefin copolymer is cured with a radical initiator. Although the patent further discloses that co-agents or combination of co-agents may be used, no distinction is drawn between the various types listed as being suitable.
In general, the prior art on peroxide curing teaches that only partially cured TPV blends are of industrial use, since compositions with the extensive crosslinking of fully cured blends exhibit undesirable processing characteristics, including decreased flowability and poor oil dispersion. As a result, the TPV industry also does not have a robust and consistent peroxide cure system to commercially produce fully cured materials with properties that are comparable to the phenolic cured TPV.
It is desired to provide a peroxide cure system to prepare TPV products with performance equal to or better than phenolic cured TPVs, especially in oil resistance and compression set.